At different stages in the life of a producing oil or gas well, it is necessary to perform interventions; for example, to effect repairs or modifications in the production tubing assembly, or for modification of the producing zones by perforating or drilling new zones of the reservoirs. One approach is to kill the well (i.e. fill it with a dense fluid to balance the pressure of fluids in the underground formation and prevent their production into the well). While this does mean that the well is under control, it is lengthy and expensive and can lead to permanent damage of the producing formation. Interventions can also be performed without killing the well and with pressure at the well head. One of the issues with interventions under pressure is the deployment of the equipment through the well head under pressure.
Various interventions are usually done to maintain the producing tubing assembly, such as:                cleaning of the well bore        pumping of stimulation or cleaning fluids        milling restrictions        operations or repairs of valves        
Furthermore, to enhance production, more zones of the reservoir can be put to production by, for example, perforating new zones and or drilling lateral holes.
It is often required to perform these interventions without killing the well to avoid damage to producing zones by the invasion of killing fluids. This means that it is necessary to keep the well under balance or near balance with produced fluids or gas or light fluids. This also often implies pressure to be present at the well head.
These interventions may also involve circulating fluids to surface through tubes or hoses that need to exit the well at surface under pressure. Circulation can be “direct” by pumping down fluids through tubes or hoses or can be “reverse” with well fluids being circulated out through the deployed tubes or hoses.
Standard well head equipment (Christmas tree) and standard well head deployment equipment do not fully address these needs to deploy cables, tubes, pipes or hoses through a well head at surface and to allow circulation of fluids through these tubes, pipes or hoses.
One current approach to perform these types of interventions is to use coil tubing, but this involves very large and expensive surface equipment such as injectors and strippers. Safety coefficients for coil tubing are low and it is not widely accepted to circulate well fluids to surface with coil tubing. Also, very high stress in coil tubing during deployment leads to fatigue and the possibility of failures not compatible with safe operation under pressure at surface. Another existing approach to perform these types of interventions is a pipe snubbing system, but this system has several limitations in terms of time performance and safety.
One proposal which allows pressure to be maintained in the well during an intervention is described in EP 1, 696,101, which describes the use of a secondary flow line housed on a drum inside a pressure vessel at the surface. The secondary line is relatively short compared to the depth of the well and is used to divert fluids while other pipe connections are being made.
The objective of the invention is to allow an easy deployment of long cables, tubes, pipes or hoses in a well under surface pressure in a timely and cost efficient manner with minimum requirements to the downhole equipment other than requirements arising generally from downhole operations. This invention also allows the production of well fluid through downhole conduit to surface safely without the need to design the conduit with the full safety coefficient usually required for surface equipment under pressure.